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Chapter 7. Control Charts for Attributes

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Chapter 7. Control Charts for Attributes

Control Chart for Fraction Nonconforming

Fraction nonconforming is based on the binomial distribution.

n: size of population

p: probability of nonconformance

D: number of products not conforming

Successive products are independent.

Mean of D = np

Variance of D = np(1-p)

Sample fraction nonconformance

w: statistics for quality

Mean of w: μw

Variance of w: σw2

L: distance of control limit from center line (in standard deviation units)

If p is the true fraction nonconformance:

If p is not know, we estimate it from samples.

m: samples, each with n units (or observations)

Di: number of nonconforming units in sample i

Average of all observations:

Example 6-1. 6-oz cardboard cans of orange juice

If samples 15 and 23 are eliminated:

Additional samples collected after adjustment of control chart:

Control chart variables using only the recent 24 samples:

Set equal to zero for negative value

Three parameters to be specified:

- sample size
- frequency of sampling
- width of control limits
Common to base chart on 100% inspection of all process output over time.

Rational subgroups may also play role in determining sampling frequency.

np Control Chart

Variable Sample Size

Variable-Width Control Limits

Variable Sample Size

Control Limits Based on an Average Sample Size

Use average sample size. For previous example:

Variable Sample Size

Standard Control Chart

- Points are plotted in standard deviation units.

UCL = 3

Center line = 0

LCL = -3

Skip Section 6.2.3 pages 284 - 285

Operating Characteristic Function and Average Run Length Calculations

Probability of type II error

Since D is an integer,

Average run length

If the process is in control:

If the process is out of control

Control Charts for Nonconformities (or Defects)

Procedures with Constant Sample Size

x: number of nonconformities

c > 0: parameter of Poisson distribution

Set to zero if negative

If no standard is given, estimate c then use the following parameters:

Set to zero if negative

There are 516 defects in total of 26 samples. Thus.

There are 516 defects in total of 26 samples. Thus.

Sample 6 was due to inspection error.

Sample 20 was due to a problem in wave soldering machine.

Eliminate these two samples, and recalculate the control parameters.

New control limits:

Additional samples collected.

Further Analysis of Nonconformities

Choice of Sample Size: μ Chart

x: total nonconformities in n inspection units

u: average number of nonconformities per inspection unit

Control Charts for Nonconformities

Procedure with Variable Sample Size

Control Charts for Nonconformities

Demerit Systems: not all defects are of equal importance

ciA: number of Class A defects in ith inspection units

Similarly for ciB, ciC, and ciD for Classes B, C, and D.

di: number of demerits in inspection unit i

Constants 100, 50, 10, and 1 are demerit weights.

µi: linear combination of independent Poisson variables

Control Charts for Nonconformities

Operating Characteristic Function

x: Poisson random variable

c: true mean value

β: type II error probability

For example 6-3

Number of nonconformities is integer.

Control Charts for Nonconformities

Dealing with Low Defect Levels

- If defect level is low, <1000 per million, c and u charts become ineffective.
- The time-between-events control chart is more effective.
- If the defects occur according to a Poisson distribution, the probability distribution of the time between events is the exponential distribution.
- Constructing a time-between-events control chart is essentially equivalent to control charting an exponentially distributed variable.
- To use normal approximation, translate exponential distribution to Weibull distribution and then approximate with normal variable

Guidelines for Implementing Control Charts

Applicable for both variable and attribute control

Determining Which Characteristics and

Where to Put Control Charts

Choosing Proper Type of Control Chart

Actions Taken to Improve Process